The present invention relates generally to systems and methods for measuring fluid levels, and more particularly to methods and structure for measuring oil level in a vehicle engine.
Accurately measuring fluid levels is important in many applications. As but one example, automatically monitoring the quality and amount of oil in a vehicle alerts drivers in a timely fashion when maintenance should be performed as dictated by the actual condition of the vehicle. Performing maintenance when it is actually required is preferred over following a predetermined, one-size-fits-all schedule that might be too long or too short for any given vehicle, depending on the way the vehicle is driven. If too long a period elapses between maintenance, a vehicle can be damaged. On the other hand, conducting maintenance when it is not needed is wasteful both in terms of labor and in terms of natural resources. For example, if a vehicle doesn""t require an oil change but nevertheless receives one, oil is in effect wasted.
Accordingly, systems have been provided for measuring various parameters of a vehicle""s engine oil, and to generate warning signals when maintenance is due as indicated by the condition of the oil. Among the parameters that are typically measured are oil temperature, condition, and level, and the signals from multiple sensors can be combined to generate the final engine oil maintenance signal. Of importance to the present invention is the accurate measurement of oil level.
As understood herein, oil level depends on many factors. Electrical properties of oil are used to measure level, which has the advantage of providing very accurate measurements. We have recognized, however, that the electrical properties depend on, among other things, temperature, oil brand, oil condition, and oil contamination, making it difficult to accurately measure oil level over a wide temperature range and independently of brand and condition.
The present invention understands that a reference sensor can be used along with a level sensor. The signal from the level sensor is divided by the signal from the reference sensor to cancel the effects of oil condition and so on, leaving a signal that represents level only. As recognized herein, two different oil levels may have the same output. In order to distinguish the two different oil levels, a fixed and known value capacitor, referred to herein as an on chip capacitor, is needed. If untrimmed, the on chip capacitor may have a twenty percent (plus or minus) variation around the nominal value. This variation is unacceptable for the required accuracy for oil level, which typically is plus or minus ten percent. The on chip capacitor may be trimmed to increase its accuracy. However, trimming the on chip capacitor increases the cost of the oil sensor.
Existing sensors also have very small signal strengths, and, hence, poor signal-to-noise ratios. Significant amplification of the output signal is required, and this in turn introduces noise amplification and the problems attendant thereto. Moreover, to eliminate temperature differential effects the two sensors should be positioned close together. However, placing the sensors close together normally dictates using a relatively low input signal frequency that might not be effective at low oil temperatures. The present invention has recognized these prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies. More specifically, the present invention can accurately determine the oil level by precalibration of the sensor without trimming the on-chip capacitor, and thus without increasing the cost of the sensor.
A method for measuring fluid level in a motor vehicle includes providing a fluid level sensor that includes a receiver tube positionable in a container of fluid such that it defines an upright orientation when it is installed in the container. The fluid level sensor also includes a level tube that is coaxially disposed in the receiver tube, a reference tube that is coaxially disposed in the receiver tube below the level tube relative to the upright orientation and a circuit that is electrically connected to the tubes for outputting a signal representative of fluid level in the container.
The method includes determining and recording a sensor output, VL/Eair, with the circuit in the level/empty configuration and the sensor in air, and then determining and recording a sensor output, VL/Rair, with the circuit in the level/reference configuration and the sensor in air. The next step is to determine and record a sensor output, VL/Etest, with the circuit in the level/empty configuration and the sensor in a test fluid, and then determining and recording a sensor output, VL/Rtest, with the circuit in the level/reference configuration and the sensor in a test fluid.
Following the above steps, the sensor is placed in a subject fluid, such as oil, and a sensor output, VL/Eoil, with the circuit in the level/empty configuration, is determined and recorded. With the sensor remaining in the subject fluid and the circuit in the level/reference configuration, a sensor output, VL/Roil, is then determined and recorded. Based on the above sensor outputs, the level of subject fluid in a container is determined.
In a presently preferred embodiment of the invention discussed below, a triangular waveform is input to the circuit. Moreover, a signal is provided to a gauge that continuously displays the subject fluid level. Alternatively, a signal is provided to a warning light that is illuminated when the subject fluid level falls below a predetermined level.
In another aspect of the present invention, a method for determining fluid level uses a fluid level sensor that includes a container, a receiver tube oriented upright in the container, a level tube in the receiver tube, a reference tube in the receiver tube, and a circuit electrically connected to the tubes for outputting a signal representative of fluid level in the container. The first step in this aspect of the present invention is to place the sensor in a fluid. Next, the sensor is configured to a level/empty configuration, and a signal is input to the circuit. A first sensor output is received. The sensor is then configured to a level/reference configuration, a signal is input to the circuit, and a second sensor output is received. Based on the first and second outputs, a signal representative of fluid level in the container is then output.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: